Non-constant envelope modulation schemes such as multi-carrier modulation schemes (e.g. orthogonal frequency division multiplex (OFDM)) and single carrier quadrature amplitude modulation (QAM)), are often realized by digitally generating these modulation signals (i.e. by computational means, typically a digital signal processor (DSP)). Such computational modulation is usefully employed in both wireless and wireline applications in which non-constant envelope modulation schemes are used. For example, a wireline application may be an xDSL transport system and a wireless (RF) application may be the 802.11a wireless LAN standard, or its variants, or Broadband Fixed Wireless Systems such as LMDS or MMDS. In such applications the data is typically scrambled, encoded, and interleaved before being modulated. In the case of wireless applications, the computational modulation is performed before the signal is fed to a digital-to-analog converter (DAC) and subsequently up-converted and amplified for wireless transmission.
Advantageously, computational modulation implementations enable more economical realizations of multi-carrier modulation and signal carrier QAM transceivers. As detailed herein, the inventor has discovered that such computational modulation environments provide a suitable framework in which to apply other pre-conditioning and/or complementary computations to the waveform before and/or after the modulation process is performed in order to achieve improved circuit performance. The terms “computational modulation” and “digitally generated modulation” are used interchangeably herein and the meaning of these terms used herein is intended to be the same viz. modulation performed by computational means.
The up-converter and power amplifier of an RF transmitter must perform the frequency shifting and amplification of the modulated carrier with a minimum of distortion. For traditional single carrier modulation schemes, this implies a reasonably low dynamic range for the up-converter and also a reasonably small power back-off (from a 1 dB compression point) for the power amplifier. However, in order to achieve a minimum of distortion in a multi-carrier OFDM or single carrier QAM modulation scheme, the up/down converters must have a very high dynamic range (i.e. they must be linear and, hence, must have a high compression point), and a large power back-off (e.g. 12 dB) for the power amplifier is required, due to the high peak-to-average power ratios encountered. Both the high dynamic range requirement and the large power back-off requirement result in a very high DC power consumption for the transmitter and this creates a disadvantage of both OFDM and QAM for wireless or wireline applications.
Known designs for the 802.11a 5 GHz wireless standard integrate the transmitter functions of scrambling, encoding, IFFT (Inverse Fast Fourier Transform) generation, modulating, up-converting, and power amplifying without directly addressing the problem of the high peak-to-average power ratio associated with OFDM modulation. There is a need, therefore, for 802.11a chip architectures that integrate the MAC, PHY and RF functions of the 802.11a, 5 GHz OFDM wireless standard and minimize the high dynamic range and large power back-off requirements. More specifically, for such wireless applications there is a need for circuitry which would enable the use of power efficient, dynamic-range limited RF circuits such as Class S power amplifiers (also referred to as Class D or Switch Mode power amplifiers) and low compression-point up-converters.
For wireline xDSL applications there is also a need to achieve greater power efficiency and it would be advantageous, for such applications, to provide means which would enable the use of a high efficiency amplifier stage (e.g. Class S).
OFDM and other related multi-carrier modulation schemes are based on repetitively assigning a multiple of symbols to a multiple of carrier frequencies and calculating the IFFT to obtain the sequential segments of the time waveform to be transmitted. A significant problem for OFDM modulation is the very high peak-to-average power ratio that may occur during the time sequence output for each IFFT operation. A peak will occur when a majority of the individual carrier frequencies line up in-phase (if a peak appears, it is unlikely that a second one will occur within the same IFFT time segment due to the relatively small number of time samples). In order to establish phase references for the demodulation decision, training tones are periodically spaced throughout the multiple of carrier frequencies.
For OFDM modulators the first few samples of the time sequence output for each IFFT operation make up a guard interval. The guard interval occurs during the time in which the multi-path channel is stabilizing. In order to have available the complete IFFT time sequence for the receiver to operate on, the first few samples are cyclically rotated and appended to the end of the IFFT. The time samples following the preamble are windowed with a weighting function in order to control frequency side-lobes. A typical weighting function is a trapezoidal waveform, having 1 time sample at both the beginning and end weighted to 0.5.
According to a known property of the Fourier transform pair, referred to as the shifting property, a shift in one domain corresponds to a complex rotation (phase shift) in the other domain. Further, for the FFT/IFFT realization, a progressive phase shift with respect to frequency in the frequency domain corresponds to a cyclic rotation of the corresponding time waveform segment.